AstraZeneca’s US launch of a fixed-dose diabetes drug combination of saxagliptin and dapagliflozin has been delayed, due to concerns of the Food and Drug Administration (FDA).

In a so-called complete response letter, the US regulatory agency asked for more clinical data on the drug cocktail that has been touted as a potential blockbuster. AstraZeneca said this could apply to data from ongoing trials or new studies, leading analysts to predict a possible launch delay of up to two years.

The individual component drugs in the new mix are already approved and marketed for the treatment of type 2 diabetes under the brand names Onglyza and Farxiga. The FDA’s action is not expected to affect their status.

Last year, AstraZeneca predicted the fixed-dose combination could generate peak annual sales of $3 billion, out of total diabetes revenue of $8 billion expected by 2023.

Sales of Onglyza, a DPP-IV inhibitor reached $391 million in the first half of 2015, with recently launched Farxiga, an SGLT2 inhibitor producing revenues of $205 million.